A broad range of optoelectronic display devices is known, ranging from incandescent lamps and light emitting diodes through to liquid crystal and plasma displays and cathode ray tubes. A particularly important category of displays is pixellated displays and, for low power applications, liquid crystal displays, (LCDs) are generally the technology of choice. Known LCD displays operate in transmissive mode, for example using a back light, reflective mode, for example illuminated by daylight, and transflective mode in which the pixels have both reflective and transmissive elements. However despite their popularity LCD displays still suffer from a number of problems which, even after many years of research, have not been fully overcome. Thus generally speaking LCD displays are relatively slow, with switching times of the order of milliseconds, and have a relatively limited viewing angle. LCDs displays can also suffer from viewing artefacts such as display inversion at certain viewing angles, and have a relatively unexciting visual appearance as compared with emissive display technologies. Furthermore since LCD displays are passive displays operating by blocking either transmitted or reflected light they have an upper efficiency limit which is reduced in colour displays by the need for coloured filters.
Emissive display technologies overcome many of the above described problems and can provide a wide viewing angle and a bright, colourful and interesting display with fewer visual artefacts. Known emissive display technologies include cathode ray tubes, plasma display panels, thin film electro-luminescent displays, and organic light emitting diodes (OLEDs), but a general problem with emissive displays is their relatively high power consumption which makes them unsuitable for many applications, and in particular, for many portable applications.
There therefore exists a general need to improve upon conventional displays and to address the above problems, particularly the problems of power consumption and viewability.
Organic light emitting diodes (OLEDs) provide many advantages over better known display technologies including ease of fabrication and flexibility in the design of new materials for displays. Organic LEDs have been know for around ten years and can be based on either conjugated polymers or on smaller molecules although, generally speaking, the main features of devices based upon both these materials are similar.
Typically an organic LED comprises a substrate on which a series of layers is deposited including a pair of electrode layers to serve as an anode and cathode and, between these, a layer of electroluminescent organic material. Optionally a hole-transporting layer is incorporated between the anode and the electroluminescent layer and/or an electron transport layer is incorporated between the electroluminescent layer and the cathode. Generally heterostructures used for inorganic LEDs can be also be adapted for organic LEDs.
In the case of polymer-based devices materials such as PPV (poly(p-phenylenevinylene)) may be used for the electroluminescent layer whilst in the case of smaller molecule devices this layer may comprise materials such as aluminium trisquinoline. The hole-transporting layer may comprise PEDOT (doped polyethylene dioxythiophene) in the polymer-based devices or triarylamines in the smaller molecule devices. The electron-transporting layer may comprise oxadiazoles in the smaller molecule devices; there is generally no electron transporting layer in the polymer devices. The anode typically has a higher work function than the cathode and is normally transparent to allow light to escape from the electroluminescent layer, often ITO (indium tin oxide) is used for the anode.
Organic LEDs switch much faster than LCDs, typically in less than one microsecond. The flexibility of organic chemistry also makes it relatively straight forward to synthesise new active materials for organic LEDs as compared with inorganic LEDs, for example to allow tuning of the organic material's semiconductor band gap. A further advantage of polymer LEDs is that they are relatively straight forward to fabricate since deposition of the active layer can be performed at room temperature using, for example, spin coating. Organic LEDs can also be formed on flexible substrates and patterned simply by pixellation of one of the electrodes.
Further details of organic LED-based devices may be found in WO90/13148, WO98/59529, WO99/48160, WO95/06400, GB 2,312,326A, and U.S. Pat. No. 5,965,901, all in the name of the present applicant, and all of which are hereby incorporated by reference.
Notwithstanding the significant advantages which organic LEDs provide over more conventional display technologies, there is still a need for lower power consumption display devices with longer lifetimes.
The present invention stems from research work carried out into organic light emitting diodes but is based upon an entirely new principle in the field of optoelectronic displays. In particular, the applicant has recognised that the electroluminescent materials normally used in organic light emitting diodes are usually also photoluminescent and that this photoluminescence may be reduced or quenched by applying an electric field to the photoluminescent material when incorporated in a suitable structure. Suitable structures include conventional OLED structures and the electric field necessary to quench the photoluminescence can be applied to the photo-(or electro-)luminescent material simply by reverse biassing the OLED device, although this photoluminescence quenching effect is difficult to observe under ordinary circumstances. The applicants have also recognised that the idea of using photoluminescence quenching to display information is not, in principle limited to the device structure and materials used for organic LEDs but could also be applied to device structures and materials used for inorganic LEDs.
WO98/41065 discloses the application of either polarity of driving voltage to an electroluminescent polymer-based display to drive either red light emission from an interface of the polymer or green light emission from the bulk of the polymer. However, in both cases, the light emitting semiconductor is forward biassed (the device effectively comprises two back-to-back diodes). U.S. Pat. No. 6,201,520 describes the use of reverse biassing for non selected pixels in a pixellated OLED display to prevent cross talk which could otherwise be caused by the (electrically) semi-excited state of the non-selected pixels. U.S. Pat. No. 5,965,901 describes the use of a pulse driving scheme for an organic light-emitting polymer device to improve device lifetime in which positive pulses are separated by negative (reverse bias) pulses. U. Lemmer et. al., Synthetic Metals, 67 (1994) 169-172 describes the experimental observation of photoluminescence quenching in an ITO/PPV/Al structure. However none of these prior art documents discloses a display based upon the photoluminescence quenching principle or the use of photoluminescence quenching to provide a display.